High fracture toughness aluminum-copper-lithium sheet or light-gauge plates suitable for fuselage panels

ABSTRACT

An aluminum alloy comprising 2.1 to 2.8 wt. % Cu, 1.1 to 1.7 wt. % Li, 0.1 to 0.8 wt. % Ag, 0.2 to 0.6 wt. % Mg, 0.2 to 0.6 wt. % Mn, a content of Fe and Si less or equal to 0.1 wt. % each, and a content of unavoidable impurities less than or equal to 0.05 wt. % each and 0.15 wt. % total, and the alloy being substantially zirconium free.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to French Application No. 0512931 filedDec. 20, 2005, U.S. Provisional Application No. 60/762,864 filed Jan.30, 2006, and PCT/FR2006/002733 filed Dec. 14, 2006, the contents ofwhich are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to aluminum alloys and moreparticularly, to such alloys, their methods of manufacture and use,particularly in the aerospace industry.

2. Description of Related Art

Continuous efforts are being directed towards the development ofmaterials that could simultaneously reduce weight and increasestructural efficiency of high-performance aircraft structures.Aluminum-lithium (AlLi) alloys are very appealing regarding this targetbecause lithium can reduce the density of aluminum by 3 percent andincrease the elastic modulus by 6 percent for every weight percent oflithium added. However, AlLi alloys have yet to be extensively used inthe aircraft industry due to several drawbacks of early generationalloys such as, for example, inadequate thermal stability, anisotropyand inadequate fracture toughness.

The history of AlLi alloys development is discussed, for example, in achapter “Aluminum-Lithium Alloys”: of the book Aluminum and AluminumAlloys, (ASM Specialty Handbook, 1994). The first aluminum-lithiumalloys (Al—Zn—Cu—Li) were introduced German inventors in the 1920s,followed by the introduction of alloy AA2020 (Al—Cu—Li—Mn—Cd) in thelate 1950s and the introduction of alloy 1420 (Al—Mg—Li) in the SovietUnion in the mid-1960s. The only industrial applications of alloy AA2020were the wings and horizontal stabilizers for RA5C Vigilante aircraft. Atypical composition for alloy AA2020 was (in weight percent) Cu: 4.5,Li: 1.2, Mn: 0.5, Cd: 0.2. There were various reasons for the limitedapplications of the AA2020 alloy, for example, the fact AA2020 exhibitedshortcomings in fracture toughness. In addition to the specific effectof Cd, the use of Mn in this alloy was assessed to be one of the reasonsof its limited properties. In 1982, E. A. Starke stated (inMetallurgical Transactions A, Vol 13A, p 2267) “The larger Mn-richdispersoids may also be detrimental to ductility by initiating voids”.This idea of a detrimental effect of Mn was broadly recognized by thoseskilled in the art. For example, in 1991, Blackenship stated (inProceedings of the Sixth International Aluminum-Lithium Conference,Garmisch-Partenkirchen, p 190), “Manganese-rich dispersoids nucleatevoids and thus encourage the fracture process”. It was suggested thatzirconium should be used instead of manganese for grain structurecontrol. In the same document, Blackenship stated, “zirconium is thealloying element of choice for grain structure control in Al—Li—X”.

The development of AlLi alloys continued in the 1980s and led to theintroduction of commercial alloys AA8090, AA2090 and AA2091. All thesealloys contained zirconium instead of manganese.

In the early 1990s, a new family of AlLi alloys containing silver knownunder the trademark “Weldalite”® was introduced. These alloys typicallycontained lower Li and exhibited better thermal stability. U.S. Pat. No.5,032,359 (Pickens, Martin Marietta) describes alloys containing from2.0 to 9.8 weight percent of an alloying element consisting of Cu, Mgand mixtures thereof, from 0.01 to 2.0 weight percent of Ag, from 0.2 to4.1 weight percent of Li and from 0.05 to 1.0 weight percent of a grainrefiner additive selected from Zr, Cr, Mn, Ti, B, Hf, V, TiB₂ andmixtures thereof. It should be noted that the list of grain refinersproposed by Pickens actually mixes elements used for foundry grainrefining (such as TiB₂) and elements used for grain structure controlduring the transformation operations such as zirconium. Even thoughPickens stated that, “although emphasis herein shall be placed upon useof zirconium for grain refinement, conventional grain refiners such asCr, Mn, Ti, B, Hf, V, TiB₂ and mixtures thereof may be used”, it clearlyappears from the history of AlLi alloy development that a prejudiceagainst the use of any element other than Zr for grain structure controlexisted to the one skilled in the art. Indeed, in all of the examplesdescribed by Pickens, Zr was used.

Use of zirconium for grain refining can also be found in an alloydeveloped more recently (AA2050, see also WO2004/106570), manganeseaddition being used to improve toughness. In AA2297, which containslithium, copper, manganese and optionally magnesium but no silver,zirconium is also used for grain refining. U.S. Pat. No. 5,234,662discloses a preferred composition of 1.6 wt. % Li, 3 wt. % Cu, 0.3 wt. %Mn and 0.12 wt. % Zr. AA2050 and AA2297 alloys have been mainly proposedfor thick plates, with a gauge higher than 0.5 inch.

Another family of AlLi alloys, which contained Zn, was described forexample in U.S. Pat. No. 4,961,792 and U.S. Pat. No. 5,066,342 anddeveloped in the early 1990s. The metallurgy of these alloys cannot becompared to the metallurgy of “Weldalite”® alloys because theincorporation of a significant amount of zinc, and in particular thecombination of zinc with magnesium, significantly modifies theproperties of the alloy, for example in terms of strength and corrosionresistance.

In order to use AlLi alloys for fuselage skin applications, the alloysshould reach the same or even better performances in strength, damagetolerance and corrosion resistance than currently used Li-free alloys.In particular, resistance to fatigue crack growth is a major concern forthose applications and that explains why alloys recognized for theirhigh damage tolerance, such as AA2524 and AA2056 alloys, aretraditionally used. Weldability and corrosion resistance are also amongother desirable properties. With the increasing trend to reduce costlymechanical fastening operations in the aircraft industry, weldablealloys such as AA6013, AA6056 or AA6156 are introduced for fuselage skinpanels. High corrosion resistance is also desirable in order tosubstitute clad products with less expensive bare products.

It was known that Al—Li alloys often have problems in terms ofanisotropy in tensile properties, which in turn, governs the extent ofanisotropy in the other mechanical properties. Low yield strength atintermediate test directions, for example 45° to the rolling direction,is a prominent manifestation of the anisotropy.

As far as damage tolerance properties are concerned, the development ofan R-Curve is a widely recognized method to characterize fracturetoughness properties. The R-curve represents the evolution of theeffective stress intensity factor for crack growth as a function ofeffective crack extension, under increasing monotonic loading. TheR-curve enables one to determine the critical load for unstable fracturefor any configuration relevant to cracked aircraft structures. Thevalues of stress intensity factor and crack extension are effectivevalues as defined in the ASTM E561 standard. The generally employedanalysis of conventional tests on center cracked panels gives anapparent stress intensity factor at fracture [K_(app)]. This value doesnot necessarily vary significantly as a function of R-curve length.However the length of the R-curve—i.e. maximum crack extension of thecurve—is an important parameter in itself for fuselage design, inparticular for panels with attached stiffeners.

There is a need for a high strength without anisotropy, high fracturetoughness, and especially high crack extension before unstable fracture,high corrosion resistance, low density (i.e. not more than about 2.70g/cm³) Al—Cu—Li alloy for aircraft applications, and in particular forfuselage sheet applications.

SUMMARY OF THE INVENTION

For these and other reasons, the present inventors arrived at thepresent invention directed to an aluminum copper lithium magnesiumsilver alloy, that is capable of exhibiting high strength withoutanisotropy, and high toughness. The present invention is also capable ofspecifically exhibiting high crack extension before unstable fracture ofwide pre-cracked panels, as well as high corrosion resistance.

By employing alloys with a low zirconium content (i.e. preferably lessthan or equal to about 0.04 wt %) it is possible to achieve hightoughness for Al—Cu—Li alloys. It is also possible to achieve anadvantageously optimized compromise between static mechanical propertiesand toughness.

Additional objects, features and advantages of the invention will be setforth in the description which follows, and in part, will be obviousfrom the description, or may be learned by practice of the invention.Objects, features and advantages of the invention may be realized andobtained by means of the instrumentalities and combination particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate a presently preferred embodimentof the invention, and, together with the general description given aboveand the detailed description of the preferred embodiment given below,serve to explain the principles of the invention.

FIGS. 1-5 are directed to certain aspects of the invention as describedherein. They are illustrative and not intended as limiting.

FIG. 1: R-curve in the T-L direction (CCT760).

FIG. 2: R-curve in the L-T direction (CCT760).

FIG. 3: Evolution of the fatigue crack growth rate in the TL orientationwhen the amplitude of the stress intensity factor varies.

FIG. 4: Evolution of the fatigue crack growth rate in the LT orientationwhen the amplitude of the stress intensity factor varies.

FIG. 5: Relative evolution of TYS when the orientation with respect torolling direction vanes.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Unless otherwise indicated, all the indications relating to the chemicalcomposition of the alloys are expressed as a mass percentage by weightbased on the total weight of the alloy. Alloy designation is inaccordance with the regulations of The Aluminum Association, known ofthose skilled in the art. The definitions of tempers are defined byEuropean standard EN 515.

Unless mentioned otherwise, static mechanical characteristics, in otherwords the ultimate tensile strength UTS, the tensile yield stress TYSand the elongation at fracture A, are determined by a tensile testaccording to standard EN 10002-1, the location at which the pieces aretaken and their direction being defined in standard EN 485-1.

The fatigue crack propagation rate (using the da/dN−ΔK test) isdetermined according to ASTM E 647. A plot of the effective stressintensity versus effective crack extension, known as the R curve, isdetermined according to ASTM standard E561. The critical stressintensity factor K_(C), in other words the intensity factor that makesthe crack unstable, is calculated starting from the R curve. The stressintensity factor K_(CO) is also calculated by assigning the initialcrack at the beginning of the monotonous load, length to the criticalload. These two values are calculated for a test piece of the requiredshape. K_(app) denotes the K_(CO) factor corresponding to the test piecethat was used to make the R curve test. K_(eff) denotes the K_(C) factorcorresponding to the test piece that was used to make the R curve test.Δa_(eff(max)) denotes the crack extension of the last point of the Rcurve, that is valid according to standard ASTM E561. The last point isobtained either when the test sample breaks or possibly when the stresson the uncracked ligament is higher than the yield stress of thematerial. Unless otherwise mentioned, the crack size at the end of thefatigue precracking stage is W/3 for test pieces of the M(T) type,wherein W is the width of the test piece as defined in standard ASTME561.

It should be noted that the width of the test panel used in a toughnesstest could have a substantial influence on the measured R curve.Fuselage sheets being large panels, only toughness results obtained onwide samples, such as samples with a width of at least 400 mm, aredeemed significant for a toughness performance evaluation in the presentinvention. For this reason, only CCT760 test samples, which had a width760 mm, were used for toughness evaluations. The initial crack lengthwas 2ao=253 mm.

The phrase “sheet or light-gauge plate” as used herein refers to arolled product not exceeding about 0.5 inch (or 12.7 mm) in thickness.

The term “structural member” as used herein refers to a component usedin mechanical construction for which the static and/or dynamicmechanical characteristics are of particular importance with respect tostructure performance, and for which a structure calculation is usuallyprescribed or undertaken. These are typically components the rupture ofwhich may seriously endanger the safety of the mechanical construction,its users or third parties. In the case of an aircraft, structuralmembers include, for example, members of the fuselage (such as fuselageskin), stringers, bulkheads, circumferential frames, wing components(such as wing skin, stringers or stiffeners, ribs, spars), empennage(such as horizontal and vertical stabilisers), floor beams, seat tracks,and doors.

An aluminum-copper-lithium-silver-magnesium-manganese alloy according toone embodiment of the invention advantageously has the followingcomposition: TABLE 1 Compositional Ranges of invention Alloys (wt. %,balance Al) Cu Li Ag Mg Mn Broad 2.1-2.8 1.1-1.7 0.1-0.8 0.2-0.6 0.2-0.6Preferred 2.2-2.6 1.2-1.6 0.2-0.6 0.3-0.5 0.2-0.5 More preferred 2.3-2.51.3-1.5 0.2-0.4 0.3-0.4 0.3-0.4Alloys of the present invention are advantageously substantiallyzirconium free. By “substantially zirconium free”, it is meant that thezirconium content shall be less than about 0.04 wt % and preferably lessthan about 0.03 wt % and still more preferably less than about 0.01 wt%.

Unexpectedly, the present inventors discovered that a low zirconiumcontent enabled an improvement in toughness of Al—Cu—Li—Ag—Mg—Mn alloys;in particular the length of the R-curve in both the T-L and L-Tdirections was significantly increased. The use of manganese instead ofzirconium for grain structure control had several additional advantagessuch as obtaining a recrystallized structure and beneficial isotropicproperties over a wide range of thicknesses from 0.8 to 12 mm or fromabout 1/32 to about ½ inch.

Fe and Si typically affect fracture toughness properties. The amount ofFe should preferably be limited to 0.1 wt. % (preferably not more than0.05 wt. %) and the amount of Si should preferably be not more than 0.1wt. % (preferably not more than 0.05 wt. %). All unavoidable impuritiesshould advantageously be limited to 0.05 wt. %. If the alloy does notinclude any additional alloying elements, the remainder is aluminum.

The present inventors found that if the copper content is higher thanabout 2.8 wt. %, the fracture toughness properties may in some cases,rapidly drop, whereas if the copper content is lower than about 2.1 wt.%, mechanical strength may be too low.

As far as lithium content is concerned, lithium content higher than 1.7wt. % leads to problems of thermal stability. A lithium content lowerthan 1.2 wt. % results in inadequate strength and a lower gain indensity.

It was also found by the present inventors that if the silver content isless than about 0.1 wt. %, the mechanical strength obtained may not meetdesired properties. The silver content should however advantageously bemaintained below 0.8 wt. % and preferably below 0.4 wt. %, to avoid anincrease of density and for cost reasons.

Extruded, rolled or forged products can be made with an alloy accordingto the present invention. Advantageously an alloy according to thepresent invention can be used to make sheet or light gauge plates.

Products according to the present invention exhibit a very high fracturetoughness performance. The inventors suspect that the absence of Zr inproducts according to the invention may be related to this performancein terms of fracture toughness. Zr and Mn, which can both be used forgrain structure control, exhibit very different behaviors. As aperitectic element, Zr is usually enriched in the grain center anddepleted at the grain boundaries, whereas Mn, which is a eutecticelement with a partition coefficient close to one, is distributed muchmore homogeneously during solidification. The different behavior of Zrand Mn during solidification might be related to their different effectsobserved in terms of fracture toughness. A recrystallized structure,which is favored here by the substantially zirconium free composition,may also by itself have a beneficial effect on toughness.Advantageously, the recrystallization rate of products according to thepresent invention is at least 80%.

The present inventors found that a homogenization temperature should bepreferentially be from 480 to 520° C. for 5 to 60 hours and even morepreferentially, from 490 to 510° C. for 8 to 20 hours. The presentinventors also observed that homogenization temperatures higher than520° C. may tend to reduce the performance in terms of fracturetoughness in some instances. The inventors believe that the technicaleffect of homogenization conditions is in relation with the describeddifferent behavior during solidification.

For sheet and light-gauge plate manufacture, the hot-rolling initialtemperature, is preferentially 450-490° C. For sheet and light gaugeplates, hot rolling is preferably carried out approximately to from 4 to12.7 mm gauge slabs. For approximately 4 mm gauge or less, a coldrolling step can optionally be added if desired for any reason. Forsheet or light-gauge plate manufacture, the sheet or light-gauge plateobtained preferably ranges from 0.8 to 12.7 mm gauge, and the presentinvention is more advantageous for 1.6 to 9 mm gauge slabs, and evenmore advantageous for 2 to 7 mm gauge slabs. A product according to theinstant invention is then solution heat treated, preferably, by soakingat 480 to 520° C. for 15 min to 4 h and quenched with room temperaturewater.

The product is then stretched from 1 to 5%, and preferentially from 2 to4%. If the stretching is higher than 5%, the mechanical properties maynot be as improved and industrial difficulties such as high ratio ofdefective parts could be encountered, which could increase the cost ofthe product. Aging is carried out at 140-170° C. for 5 to 80 h, and morepreferentially at 140-155° C. for 20-80 h. Lower solution heat-treatingtemperatures generally favor high fracture toughness. In one embodimentof the present invention comprising a welding step, the aging step canbe divided into two steps: a pre-aging step prior to a weldingoperation, and a final heat treatment to form a welded structuralmember.

Characteristics of the sheets and light-gauge plates obtained with thepresent invention include one or more of the following:

-   The tensile yield strength in the L-direction is preferably at least    390 MPa or even 400 MPa.-   The ultimate tensile strength in the L-direction is preferably at    least 410 MPa or even 420 MPa.-   The tensile yield strength at 45° to the rolling direction is at    least equal to the tensile yield strength in the LT direction.-   The difference between the tensile yield strength at 45° to the    rolling direction and the tensile yield strength in the LT direction    as defined by (TYS (TL)−TYS (45°))/ TYS (TL) is between +5% and −5%    and preferably between +3% and −3%.-   The fracture toughness properties using CCT760 (2ao=253 mm)    specimens include one or more of the following:    -   K_(app) in T-L direction is preferably at least 100 MPa√{square        root over (m)}, and preferentially at least 120 MPa√{square root        over (m)};    -   K_(app) in L-T direction is at least 150 MPa√{square root over        (m)}, and preferentially at least 160 MPa√{square root over        (m)};    -   K_(eff) in T-L direction is at least 120 MPa√{square root over        (m)}, and preferentially at least 150 MPa√{square root over        (m)};    -   K_(eff) in L-T direction is at least 160 MPa√{square root over        (m)}, and preferentially at least 220 MPa√{square root over        (m)};    -   Δa_(eff (max)), the crack extension of the last valid point of        the R-curve in T-L direction is preferably at least 60 mm, and        preferentially at least 80 mm;    -   Δa_(eff (max)) from R-curve in L-T direction is preferably at        least 60 mm, and preferentially at least 80 mm.

The terms high strength, high fracture toughness, high crack-extensionbefore unstable fracture, low anisotropy as used herein refer toproducts displaying one or more of the properties mentioned above.

Advantageously, the recrystallization rate of the sheets or light gaugeplates according to the invention is at least about 80%.

Forming of products of the present invention may advantageously be madeby stretch-forming, deep drawing, pressing, spinning, rollforming and/orbending, these techniques being known to persons skilled in the art. Forthe assembly of the structural part, all known and possible adhesivebonding, riveting and welding techniques suitable for aluminum alloyscan be used if desired. The products may be fixed to stiffeners orframes, for example, by adhesive bonding, riveting or welding. Theinventors have found that if welding is chosen, it may be preferable touse low heat welding techniques, which helps ensure that the heataffected zone is as small as possible (is minimizing). In this respect,laser welding and friction stir welding often give particularlysatisfactory results.

Products of the present invention, before and/or after forming, mayadvantageously be subjected to artificial aging to impart improvedstatic mechanical properties. This artificial aging may also beconducted in any advantageous manner on an assembled structural part ifdesired. Products of the invention can advantageously be used for themanufacture of structural members for aeronautical construction. Astructural part can be formed of a sheet or light-gauge plate accordingto the present invention and of stiffeners and/or frames. Stiffeners orframes are preferably made of extruded profiles. Structural parts may beused for example and in particular for airplane fuselage panelsconstruction as well as for any other use where the instant propertiescould be advantageous.

The present inventors found that products of the invention haveparticularly favorable compromise between static mechanical properties,fracture toughness and density. For known low-density products, the hightensile and yield strengths sheet or light-gauge plates generally have alow fracture toughness. For the sheet or light-gauge plate of theinvention, the high fracture toughness properties, and in particular thevery long R-curve properties favor industrial application for aircraftfuselage skin parts. Some embodiments of the present invention havedensities of not more than about 2.70 g/cm³ even not more than 2.69g/cm³ and even more preferably of not more than about 2.66 g/cm³.

Products of the invention generally do not raise any particular problemsduring subsequent surface treatment operations conventionally used inaircraft manufacturing, in particular for mechanical or chemicalpolishing, or treatments intended to improve the adhesion of polymercoatings.

Resistance to intergranular corrosion of products of the presentinvention is generally high; for example, typically only pitting isdetected when the metal is submitted to corrosion testing. In apreferred embodiment of the invention, the sheet or light-gauge plate ofthe invention can be used without cladding on either surface with a lowcomposition aluminum alloy.

These as well as other aspects of the present invention are explained inmore detail with regard to the following illustrative and non-limitingexample:

EXAMPLE

The inventive example is labeled C. Examples B and D do not include Agare presented for comparison purposes. Sample D has a Cu content outsidethe invention as well. Example A is a reference AA2098 silver containingalloy and employs Zr as opposed to Mn for grain structure control andemploys high Cu. The chemical compositions of the various alloys testedare provided in Table 2. TABLE 2 Chemical composition (weight %) Castreference Si Fe Cu Mn Mg Cr Zn Zr Li Ag Ti A (2098) 0.03 0.04 3.6 0.010.32 0.01 0.01 0.14 1.0 0.33 0.02 B 0.03 0.04 2.2 0.29 0.3 — — <0.01 1.4— 0.02 C 0.03 0.03 2.4 0.29 0.3 — — <0.01 1.4 0.34 0.02 D 0.28 0.03 1.50.28 0.3 — — <0.01 1.4 — 0.03

The density of the different alloys tested is presented in Table 3.Samples B to D exhibit the lowest density of the different materialstested. TABLE 3 Density of the alloys tested Density Reference (g/cm³) A(2098) 2.70 B 2.64 C 2.64 D 2.62

The methods used to manufacture the different samples are presented inTable 4. TABLE 4 Conditions of the consecutive steps of transformationReference A References B, C and D Temper T8 T8 Stress Yes Yes relievingby heating Homogenizing 8 h at 500° C. + 12 h at 500° C. 36 h at 526° C.Hot-rolling 485° C. 450 to 490° C. initial temperature Hot rollingThickness >4 mm Thickness >4 mm. Hot rolling exit temperature <280° C.Cold rolling Thickness <4 mm Thickness <4 mm, optional intermediateannealing Solution heat 2 h at 521° C. 1 h at 500° C. treating QuenchingWater at room temperature Water at room temperature Stretching 1-5%permanent set 1-5% permanent set Aging 14 h at 155° C. (4.5 mm) 48 h at152° C. 18 h at 160° C. (6.7 mm)

The grain structure of the samples was characterized by microscopicobservation of cross sections after anodic oxidation, under polarizedlight or after chromic etching. A recrystallization rate was determined.The recrystallization rate is defined as the surface fraction ofrecrystallized grains. The recrystallization rate was 100% for samplesB, C and D. For samples A#1 and A#2, the recrystallization rate was lessthan 20%.

The samples were mechanically tested to determine their staticmechanical properties as well as their resistance to crack propagation.Tensile yield strength, ultimate strength and elongation at fracture areprovided in Table 5. TABLE 5 Mechanical properties of the samples Ldirection LT direction 45° direction UTS TYS E UTS TYS E UTS TYS ESample Thickness (MPa) (MPa) (%) (MPa) (MPa) (%) (MPa) (MPa) (%) A#1 4.5573 549 11.0 559 528 12.0 A#2 6.7 559 537 11.3 553 529 10.9 494 459 15.3B 5 409 373 14.2 396 344 13.2 398 348 14.0 C 5 439 414 14.0 434 386 11.9433 387 13.1 D 5 295 228 15.8

The static mechanical properties of the samples according to theinvention are comparable to conventional damage tolerant 2XXX seriesalloy, lower than high strength alloys such as 7475 or 2098 (as testedin Sample A). The strength of the comparison alloy B was lower than thatof the alloy according to the invention (C), which might be related tothe absence of silver in the comparison alloy B. The inventors believethat the lower copper content and the lower zirconium content of thesample according to the invention explains the lower strength comparedto 2098 alloy (sample A). Anisotropy was very low for sample C accordingto the invention as shown in FIG. 5, which shows the relative evolutionof TYS when the orientation with respect to rolling direction varies.Thus, the difference between the tensile yield strength at 45° to therolling direction and the tensile yield strength in the LT direction asdefined by (TYS (TL)−TYS (45°))/TYS (TL) was −0.3% for sample C whereasit was 13.2% for the reference sample A (AA2098).

Moreover, sample C according to the invention exhibits high fracturetoughness properties. R-curves of samples A#1, B and C are provided inFIGS. 1 and 2, for T-L and L-T directions, respectively. FIG. 1 clearlyshows that the crack extension of the last valid point of the R-curve(Δ_(aeff(max))) is much larger for samples from the invention than fromsample A#1 and B. This parameter is at least as critical as the K_(app)values because, as explained in the description of related art, thelength of the R-curve is an important parameter for fuselage design.FIG. 2 shows the same trend, but the difference is smaller because theL-T direction intrinsically gives better results. Table 6 summarizes theresults of toughness tests. TABLE 6 Results of toughness tests T-L (760mm wide L-T (760 mm wide specimen) specimen) Thickness K_(app) K_(eff)K_(app) Sample [mm] (MPa√m) (MPa√m) (MPa√m) K_(eff) (MPa√m) A#1 4.5 154174 148 188 A#2 6.7 103 112 123 143 B 5.0 143 209 161 232 C 5.0 143 200172 247

The results originating from the R-curve are grouped together in Table7. Crack extension of the last valid point of the R-curve is higher forinvention sample C than for reference sample A#1. The inventors believethat several reasons can be proposed to explain this performance,unexpectedly the absence of Zr could be a major contributor, directly orindirectly, to the performance in fracture toughness. TABLE 7 R-curvesummary data Δa [mm] 10 20 30 40 50 60 70 80 K_(r) A#1 125 161 — — —(T-L direction) B 102 128 147 162 176 188 199 210 [MPa√m] C 101 130 150166 179 190 200 209 K_(r) A#1 115 141 159 174 185 (L-T direction) B 106139 162 181 197 211 224 236 [MPa√m] C 123 154 177 196 212 227 241 254

FIGS. 3 and 4 show the evolution of the fatigue crack growth rate in theT-L and L-T orientation, respectively, when the amplitude of the stressintensity factor varies. The width of sample was 400 mm (CCT 400specimen) and R=0.1. No major difference was observed between samples A,B and C. Sample C fatigue crack propagation rate is on the same range astypical values obtained for AA6156 and AA2056 alloys.

Resistance to intergranular corrosion of the samples A#1, B and C wastested according to ASTM G110. For each sample, no intergranularcorrosion was detected. Therefore, resistance to intergranular corrosionwas, high for the samples according to the present invention.

Additional advantages, features and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details, and representativedevices, shown and described herein. Accordingly, various modificationsmay be made without departing from the spirit or scope of the generalinventive concept as defined by the appended claims and theirequivalents.

All documents referred to herein are specifically incorporated herein byreference in their entireties.

As used herein and in the following claims, articles such as “the”, “a”and “an” can connote the singular or plural.

In the present description and in the following claims, to the extent anumerical value is enumerated, such value is intended to refer to theexact value and values close to that value that would amount to aninsubstantial change from the listed value.

1. An aluminum alloy comprising 2.1 to 2.8 wt. % Cu, 1.1 to 1.7 wt. % Li, 0.1 to 0.8 wt. % Ag, 0.2 to 0.6 wt. % Mg, 0.2 to 0.6 wt. % Mn, a content of Fe and Si less or equal to 0.1 wt. % each, and a content of unavoidable impurities less than or equal to 0.05 wt. % each and 0.15 wt. % total, and the alloy being substantially zirconium free.
 2. An aluminum alloy according to claim 1, comprising 2.2 to 2.6 wt. % Cu, 1.2 to 1.6 wt. % Li, 0.2 to 0.6 wt. % Ag, 0.3 to 0.5 wt. % Mg, and 0.2 to 0.5 wt. % Mn.
 3. An aluminum alloy according to claim 1 comprising 2.3 to 2.5 wt. % Cu, 1.3 to 1.5 wt. % Li, 0.2 to 0.4 wt. % Ag, 0.3 to 0.4 wt. % Mg, and 0.3 to 0.4 wt. % Mn.
 4. An aluminum alloy according to claim 1 wherein zirconium is less than or equal to 0.03 wt. %.
 5. An aluminum alloy according to claim 1, consisting essentially of the recited elements in the recited proportions.
 6. An extruded, rolled and/or forged product comprising an alloy according to claim
 1. 7. A product according to claim 6 wherein the recrystallization rate is at least 80%.
 8. A rolled product according to claim 6 wherein the thickness thereof does not exceed about 0.5 inch.
 9. A method for producing an aluminum alloy sheet or light gauge plate having high fracture toughness and strength, said method comprising: (a) casting an ingot comprising 2.1 to 2.8 wt. % Cu, 1.1 to 1.7 wt. % Li, 0.1 to 0.8 wt. % Ag, 0.2 to 0.6 wt. % Mg, and 0.2 to 0.6 wt. % Mn, a content of Fe and Si less than or equal to 0.1 wt. % each, and a content of unavoidable impurities less than or equal to 0.05 wt. % each and 0.15 wt. % total, and wherein said alloy is substantially zirconium free, (b) homogenizing said ingot at 480-520° C. for about 5 to about 60 hours, (c) hot rolling said ingot to a slab, with an hot rolling initial temperature of about 450° C. to about 490° C. and optionally cold rolling said slabs, (d) solution heat treating said slabs at about 480° C. to about 520° C. for about 15 min. to about 4 hours, (e) quenching said slabs, (f) stretching said slabs with a permanent set from about 1 to about 5%, (g) aging said slab by heating at about 140° C. to about 170° C. for about 5 to about 80 hours.
 10. A method according to claim 9 wherein said ingot consists essentially of the recited elements.
 11. A method according to claim 9, wherein the thickness of said sheet or light gauge plate is from 0.8 mm to 12.7 mm.
 12. A rolled product produced by a method of claim 9, wherein said rolled product comprises (a) a tensile yield strength in the L-direction of at least 390 MPa, and preferably at least 400 MPa, (b) a difference between the tensile yield strength at 45° to the rolling direction and the tensile yield strength in the LT direction as defined by (TYS (TL)−TYS (45°))/ TYS (TL) from +5% to −5%, (c) a plane stress fracture toughness K_(app), measured on CCT760 (2ao=253 mm) specimens, of at least 100 MPa√{square root over (m)}, (d) and/or a crack extension of the last valid point of the R-curve Δa_(eff(max)), in the T-L direction of at least 60 mm, and preferentially at least 80 mm.
 13. An aircraft fuselage panel comprising at least one rolled product according to claim
 12. 14. A structural member for aeronautical construction comprising at least one product according to claim
 6. 15. An aluminum alloy according claims 1 wherein said zirconium is present an amount of not more than about 0.04 wt %.
 16. A method claim 9 wherein zirconium is present in an amount of not more than about 0.04 wt %.
 17. A product according to claim 6, wherein zirconium is present in an amount of not more than about 0.04 wt % in said alloy.
 18. A fuselage panel according to claim 13, wherein zirconium is present in an amount of not more than about 0.04% in said alloy.
 19. A structural member according to claim 14, wherein zirconium is present in an amount of not more than about 0.04% in said alloy.
 20. A substantially Zr free AlLi alloy comprising Cu from 2.1-2.8, Ag from 0.1-0.8 and Mn from 0.2-0.6 such that wherein said alloy is formed into a sheet or light gauge plate, said sheet or light gauge plate has improved fracture toughness: Δa_(eff(max)), in the T-L direction of at least 60 mm and for which no intergranular corrosion is observed under a test according to ASTM G110.
 21. A sheet or light gauge plate comprising an alloy of claim
 20. 22. An extruded, rolled and/or forged product comprising an alloy of claim
 20. 23. An alloy of claim 20 wherein Zr is not more than 0.04%.
 24. A structural member suitable for aeronautical construction comprising an alloy of claim
 20. 25. An aeronautical component comprising an alloy of claim
 20. 26. An aluminum alloy according to claim 1 wherein zirconium is less than or equal to 0.01 wt. %.
 27. A method according to claim 11, wherein said thickness is from 1.6 mm to 9 mm.
 28. A rolled product of claim 12, wherein said difference is from +3% to −3%, said plane stress fracture toughness is at least 120 MPa√{square root over (m)} in the T-L direction and said crack extension is at least 80 mm.
 29. A substantially Zr free AlLi alloy comprising Cu from 2.1-2.8, Ag from 0.1-0.8 and Mn from 0.2-0.6, such that when said alloy is formed into a sheet or light gauge plate, said sheet or light gauge plate has improved properties as compared to a sheet or plate formed of AA2098 such that: the density of said sheet or plate is at least 2% lower than an AA2098 sheet or plate and, Δa_(eff(max)), in the T-L direction thereof is at least 50% higher than said AA2098 sheet or plate, and the ratio (TYS (L)−TYS (45°))/ TYS (L) thereof is at least 40% lower than said AA2098 sheet or plate 